Towers for wind turbines may be constructed of steel or concrete. Whilst wind turbine towers manufactured by concrete are relatively cost efficient they have a significantly higher mass. For example, a rotor having three rotor blades and a diameter of 60 m is supported by a nacelle having a total mass of approximately 207 t. The hub height of the rotor is approximately 50 m. A concrete tower suitable for such rotor has a mass ranging from approximately 430 t to approximately 540 t. In contrast thereto, the mass of a steel tower for supporting such rotor can range from approximately 87 t to approximately 114 t. However, the manufacturing costs associated with a steel tower are approximately twice the costs associated with manufacturing a concrete tower. Steel towers are preferred when the ground cannot carry the foundations and mass of a concrete tower.
Steel towers for wind turbines are generally constructed as hollow steel towers made of prefabricated tower segments. The tower is constructed or assembled by successively lifting and placing a series of tower segments on top of each other, whereby the lowest tower segment is mounted on a foundation, usually made of concrete, and includes access means such as a doorway to later allow access for a maintenance crew. Generally, the tower segments become successively smaller in diameter towards the top of the tower. Neighbouring tower segments may be connected together by means of fasteners such as construction bolts inserted into connection holes in the end flanges. For example, for a pair of tower segments comprising a ‘lower’ tower segment and an ‘upper’ tower segment, the connection holes of the neighbouring segments are positioned such that connection holes in the top flange of the lower tower segment match the connection holes in the bottom flange of the upper tower segment.
A typical wind turbine support has thus two structural main elements, the foundation and the tower. The foundation may be designed in various ways, such as a reinforced concrete spread footing, as it is preferred as a cost effective solution. This type of foundation is very suitable for sites where the ground provides a good bearing capacity. The interfaces to the steel towers are often bolted connections.
The wind turbine steel tower is subjected both to dynamic and static loads. A static load is the mass of the rotor and nacelle acting essentially in vertical direction. Another static load is the wind pressure acting essentially in horizontal direction. Dynamic loads are generated for example by the rotation of the rotor and distortions of the air flow when passing the tower which lead to a locally reduced pressure on the rotor blade passing the tower. Since the wind turbine and the associated tower are a dynamically loaded structure, the natural frequency of the tower has a major influence on the loading of the turbine and tower itself. Therefore, the tower must be partly dimensioned to avoid certain natural frequencies corresponding to the rotational speed of the rotor and the frequencies of other structure elements besides to fatigue and static loading. Wind turbine systems generally, as well as static and dynamic loads of wind turbine towers are described in Windkraftanlagen, Robert Gasch and Jochen Twele Eds., 6th edition 2010, Vieweg+Teubner.
A traditional steel tower is a tubular welded steel tower with conical, cylindrical or both shapes. These steel towers comprise shells welded or bolted together into a full tower or several tower segments as mentioned before. Traditionally, the steel type S355 according to EN 10025 is used throughout the tower.